The high pressure and low temperature of special environment in deepsea will lead to change of the property of hydraulic oil. The change of hydraulic oil will seriously affect the efficiency of gear pump as a working medium. In order to study the influence of the deep-sea environment on the efficiency of the gear pump, the internal flow field model of the gear pump is established by using the Computational Fluid Dynamics (CFD) method. The efficiency of gear pump with different sea-depth was studied. The results indicate that the efficiency of gear pump decreases with the increase of sea-depth. The influence of environmental pressure on the efficiency is higher than that of sea temperature, and the pressure of that is more obvious. With the increase of seadepth, the effect of environmental pressure on the efficiency is more obvious when the rotational speed is more than 1 900 r/min, the efficiency is reduced by about 6% and when the rotational speed is less than 1 900 r/min, the efficiency is reduced by only about 3%. The effect of temperature on the efficiency is obvious when the rotational speed is less than 1 900 r/min, the efficiency is reduced by about 4%, and when the rotational speed is higher than 1 900 r/min, the efficiency is reduced by only about 2.5%.